“I asked Chirlane, ‘Do you know what you’ll be wearing on election night?’” Ms. Kuan said in an interview. “She said she’s too busy to think about it, and she’ll wear something from her closet. So I said, ‘We’ll meet, and I’ll make a dress for you.’”

Spokespeople for Ms. McCray said the dress appeared to be one of Ms. Kuan’s but could not confirm with Ms. McCray Tuesday night.

Ms. Kuan said she custom fit the ponte knit red dress for Ms. McCray. She said the wholesale price is $92 but the dress will retail for $195 and be available in February 2014.

The designer said she believes Ms. McCray tries to support local designers and to wear color. “Color is her thing,” Ms. Kuan said.

Ms. Kuan voted for Ms. McCray’s husband for mayor on Tuesday. “It was a natural choice,” she said.

Corrections & amplifications: An earlier version of the article said the dress will be available later this year. It will be available in February 2014.